The present invention relates to apparatus for controlling the operation of a solvent programmer that produces solvent mixtures for introduction to a liquid chromatograph; and more particularly, it relates to a controller that controls the operation of a solvent programmer to produce solvent mixtures comprised of two solvents proportioned on the basis of the portion of time within a defined interval that each solvent is accessed by an appropriate valve mechanism.
A common technique utilized to separate, isolate and identify the components of mixtures is that of chromatography. Basically, chromatography concerns a process that identifies the components of a mixture on the basis of differences in rates at which the individual components of the mixture migrate through a stationary medium under the influence of a mobile phase.
One particular type of chromatography is liquid adsorption chromatography frequently utilized in the separation of organic mixtures. In liquid adsorption chromatography, the stationary phase consists of a tubular column packed with an adsorbent material, with the sample being carried through the column by a solvent commonly referred to as the "carrier". The carrier is introduced into a chromatographic instrument by a pump. An equilibrium is established for the individual components of the sample according to its "attraction" to the stationary phase and its solubility in the carrier liquids. The rate at which a solute passes through the column of the chromatograph is dependent upon the equilibria existing, and separations occur where the distributions differ.
It is important in any type of liquid chromatography that a predetermined solvent percent or ratio be precisely maintained in the mixture that is presented to the chromatograph. Also, it is sometimes desired that the percentage of solvent in the mixture be varied with time in accordance with some predetermined gradient, which is often times referred to as a solvent program.
Although numerous techniques have been in use in past, a new technique for selectively producing a wide variety of programmed solvent mixtures for use in liquid chromatographic analysis has recently been developed. In accordance with this new technique, two solvents are mixed in prescribed proportions by a solvent programmer apparatus and made available to the inlet of a chromatographic analyzer. The solvent programmer apparatus produces the desired solvent mixture by proportioning the solvents on a time proportioned basis within a defined duty cycle. This new technique is disclosed in copending U.S. application Ser. No. 768,901 and assigned to the assignee of the instant application.
Solvent programmer apparatus in accordance with that disclosed in the referenced copending application comprises two modules. One is the solvent module which contains a valve mechanism adapted to access a separate liquid solvent from each of a plurality of sources, and be opened to any one of the sources. The other module is the control module containing a controller that is operably connected to the valve mechanism in the solvent module. The controller controls the valve mechanism to perform the various solvent programs, by operating the valve mechanism to access each of the solvent sources for a predetermined portion of a duty cycle of a prescribed duration of time.
Referring to FIG. 1, a solvent programmer 10 of the type described above is presented in diagram form. Two solvents labelled solvent "A" and solvent "B", generally designated by the reference numerals 12 and 14 respectively, are mixed in the proportion called for by the solvent programmer. The ratio of one solvent to the other is controlled by the action of a two-way valve 16 operated on a prescribed duty cycle. Either a fixed percentage of B running continuously, known as "isochratic" or "single solvent" operation, or a time varying percentage of B, known as "gradient" operation, may be derived. When a time-varying program is selected, the proportion of B in the mixture can be made to increase at a fixed rate, i.e., linear gradient, or at an increasing rate, i.e., convex gradient, or at a decreasing rate, i.e., concave gradient, by appropriate control of valve 16 during sequential duty cycles. In addition, the curvature of the two non-linear rates can be selected for several shapes between concave and convex.
As stated, the solvent programmer 10 consists of two modules, a control module 18 containing a controller necessary to operate valve 16 in the desired manner, and a solvent module 20. The solvent module contains the valve 16, a pump 22 for drawing solvent from each solvent source upon being accessed by the valve, a mixer 24, and a breather reservoir 26. Additional details of the solvent module 20 may be had by reference to copending application Ser. No. 769,901.
Referring next to FIG. 2, the various linear and non-linear time-varying solvent programs that can be carried out by the solvent programmer 10 are presented in a graph relating percentage of solvent B to time in minutes. As shown in the graph, a linear gradient program is provided by a constant rate of change of the percentage of solvent B. A convex gradient is formed by an increasing rate of change of percentage of solvent B, and a concave gradient by a decreasing rate of change. Each of the non-linear gradients shown in the graph represents a particular percentage of non-linearity. When a non-linear program is run, the elasped time from 0 to 100% B is approximately the same as for the linear gradient and is a function of the percentage of non-linearity chosen.
In isochratic operation, the valve mechanism is operated, within each successive duty cycle, to proportion the solvents in constant, non-varying proportions. To achieve this, the valve mechanism is opened to each solvent for the same portion of the duty cycle, during each duty cycle. Accordingly, for a mixture having 10% solvent B and 90% solvent A, during each duty cycle, the valve mechanism would by open to solvent A 90 percent of the duty cycle and to solvent B 10 percent of the duty cycle.
In linear gradient operation, the valve mechanism is operated to proportion the solvents in amounts that vary at a fixed rate of change over time in accordance with a program. To achieve this type of operation, the valve mechanism is opened to each solvent for a different portion of the duty cycle from one duty cycle to a later occurring one. Changes in the portion of a duty cycle that the valve mechanism is opened to each solvent are at a fixed rate as between duty cycles spaced apart in time. Accordingly, for a mixture that increases in the percentage of solvent B at a rate of 5% per minute, a duty cycle occurring one minute after a previous duty cycle will have the valve mechanism opened to solvent B for 5 percent more of the duty cycle time than it was for a previous duty cycle.
In non-linear gradient operation, the valve mechanism is operated to proportion the solvents in proportions that vary at a non-constant rate of change. The rate of change may be either increasing or decreasing. In this type of operation, the valve mechanism is opened to each solvent for a different portion of the duty cycle, from one duty cycle to a latter occurring one. However, changes in the portion of a duty cycle that the valve mechanism is opened to each solvent varies at a non-constant rate as between duty cycles spaced apart in time. That is, the amount of time within a duty cycle that the valve mechanism is opened to solvent B will change non-uniformly between duty cycles spaced apart in time. Accordingly, the apparatus of the instant invention can provide a solvent mixture that varies in the rate of change of one solvent to another over time.
In addition, the controller may operate the valve mechanism in the non-linear gradient mode to effect varying degrees of non-linearity in the program run by the apparatus.
The various solvent programs discussed above and illustrated in FIG. 2 are made possible by the control signals supplied by controller 28 (see FIG. 1) in control module 18. Controller 28 opens valve 16 to each of the solvent sources, A or B, on a time proportioned basis to produce the predetermined mixture of solvents. It was disclosed that, in a preferred embodiment, the valve mechanism is a multiple-way solenoid valve and the controller comprises electronic circuitry that provides the necessary electrical control signals to the solenoid valve.
Accordingly, the instant invention seeks to provide a suitable controller for use in a solvent programmer of the type disclosed in the referenced copending application.